Modeling Damage to Limestone Exposed to Atmospheric Pollutants

Abstract

ABSTRACTPreservation of building and monument stone exposed to acidic environments relies on the understanding of acidic precipitation deposition processes and damage mechanisms. Presented here is a model which predicts sulfur accumulation in porous limestone subjected to dry deposition of SO2. The model assumes deposition and reaction of SO2 to form a thin gypsum crust on the moist surface of the stone, and subsequent sulfur (as aqueous sulfate) transport and accumulation in the stone interior driven by diurnal wetting and drying of the stone surface. Characterization of the limestone pore structure contributes significantly to the evaluation and interpretation of modeled sulfate transport and accumulation in porous building materials. Predicted sulfur accumulation in the stone interior is dependent on the surface boundary conditions, the stone pore geometry and structure, and the rates and mechanisms of aqueous/solid sulfur partitioning (i.e. adsorption, precipitation and dissolution). Model results are compared to moisture content and sulfur accumulation measured in limestone briquettes exposed to a natural dry deposition environment. The model successfully predicts moisture transport in field-exposed limestone, but overestimates the rate of sulfur accumulation. The model may be improved by quantification of the time dependence of the surface sulfate concentration and better understanding of the sulfate partitioning mechanisms.